It
has been said that the fixation of carbon dioxide by using microalgae is one of
promising methods to reduce the so-called greenhouse effects. For this purpose,
it is necessary to innovate high-performance photobioreactors for
mass-production use. However, it seems not-so-easy to attain the productivity
accomplished by laboratory-scale reactors. This is partly due to the thickness
of reactor chambers and high concentration of algae, which are simply for
accommodating necessary volume of cultivates in a reasonably economical
mass-production system, but prevent the algae from receiving sufficient
sunlight and the cultivate medium from mixing nutrients and dissolved gasses
equally.

We focused on the Bio-dome
developed by one of the authors (SH), which is a discrete-type reactor for mass
production and was adopted for commercial use recently. Firstly, the flow in a
Bio-dome was elucidated by using flow-visualization techniques. Then, the shape
of a photobioreactor was analyzed. We classified its functions into three, i.e.
sunlight-reception, global mixing, and local mixing. While the first and third
were mathematically modeled, the second was numerically simulated by using
two-phase CFD (computational fluid dynamics). Here we were interested in the
third function of photobioreactors; the local mixing, in other words, vortices
mixing algae in the thickness direction of a reactor chamber. Our mathematical
model for the dark-light cycle for photosynthesis can predict the increase of
productivity by intensifying these vortices, although the model needs further
investigation to acquire sufficient accuracy.

Next, we tried to innovate
novel forms of photobioreactors. It is important for mass production that the system
of reactors must be simple. Our design policy is that steady flow should
provide algae with equal opportunity for sunlight, nutrients, and dissolved
gases. Apparatus for unsteady flow, which is regarded as effective to reduce
unfairness among algae in terms of sunlight-reception, sometimes cause small
troubles. However, this is serious, if small, for mass production using numbers
of discrete reactors. To select better shapes of reactor chamber, the CFD code
was used to evaluate the global mixing performance. Here we introduce two
parameters; deviations of random numbers initially set in each cell of
computational mesh and of light intensity added in each cell at every
computational time-step depending on the distance from the reactor surface.
Another kind of selection was done in terms of the sunlight-reception
capacity.Two types, which had
survived through the above selections, were manufactured and tested by using Chlorococum littorale for the 12 mostly
clear winter days in Iwata. The winner of this last selection performed
0.15g/L/day (6.8g/m2/day) at the alga-concentration of 1-2.5g/L,
that is 57% more productivity than the Bio-dome.

Lastly, we consider proposing
a reasonably economical CO2 fixation system by using high-performance
photobioreactors (0.5g/L/day, 24g/m2/day) and Botryococcus braunii. The system is supposed to be constructed in
the Hobuqi Desert in China, using water from the Huang He River and exhausts
from a coal power plant in Paotou City. Our calculation shows that the total
CO2 fixation cost is about 100yen($0.8)/kg-CO2. The reduced CO2 is the one
supposed to be exhausted from current Chinese power generators burning coal,
which is replaced by Botryococcus braunii
as biomass fuel. Here, the exhaust gas from the coal power plant is directly
lead to the photobioreactors, so that the used Botryococcus braunii should be genetically manipulated to endure
sulfuric acid and nitrogen oxides.